The present invention relates to a method of compressing fibrous bodies which are held in place on a perforated support by means of negative pressure, and to an arrangement for implementing the method.
In the manufacture of absorbent articles, such as diapers, incontinence pads, sanitary towels, panty liners and the like, use is commonly made of fluff pulp, with or without the addition of superabsorbents, bonding fibres or other materials, as the absorbent layer. The fluff pulp is supplied to a mat former or the like and is given the desired shape. The shaped pulp body is then compressed so as to obtain the desired absorption properties. The pulp bodies are often conveyed on perforated conveying tracks and are held in place on the conveying tracks by means of negative pressure. In the case of continuous process lines for the manufacture of absorbent articles, a number of individual absorbent bodies or layers of absorbent material are often put onto a running web of surface layer material, and it is of course of utmost importance for manufacturing accuracy that the individual bodies or layers end up in the correct place on the running web.
One problem associated with compressing pulp bodies which are held in place by means of negative pressure on a perforated support is that, during compression, the fluff pulp is pressed into the perforations and remains in these after the compressed pulp body has been removed from the support. This problem is particularly marked when pulp bodies are compressed hard, that is to say pulp bodies are compressed to a density  greater than 200 kg/m3, which means that there is a great risk of the perforations in the support being clogged or blocked after only one or a few compressions. After the perforations of the support have become clogged, it must be cleaned in order to be capable of being used again in the process line, which leads to operational stoppages or to the introduction of complicated cleaning arrangements into the process line. Clogging also means that the pulp bodies can move on the support, which results in defective products.
The object of the present invention is to eliminate completely the risk of blocking of the perforations in the support when pulp bodies are compressed.
According to the invention, this object is achieved by means of a method of compressing fibrous bodies which are held in place on a perforated support by means of negative pressure, characterized in that each fibrous body to be compressed is first arranged on a first support which comprises parts with and without perforations within the region covered by the fibrous body, in that a first compression of the fibrous body is brought about in those portions thereof which cover the part or parts without perforations of the first support, in that the partly compressed fibrous body is then transferred to a second support which comprises perforations in the part or parts covered by compressed portions of the fibrous body but is without perforations within one or more parts covered by uncompressed portions of the fibrous body, in that a second compression of the fibrous body is brought about when the latter is placed on the second support in those portions of the body which cover the part or parts without perforations of the second support, after which, if required, similar compression steps are repeated until all portions to be compressed of the fibrous body have been compressed.
In a preferred embodiment, the supports consist of running conveying tracks, and the fibrous bodies are retained on the first support in two lateral regions extending in the running direction and are retained on the second support in a central region extending in the running direction.
The invention also relates to an arrangement for compressing fibrous bodies, characterized by a first conveying track which is perforated in one or more regions extending in the running direction of the conveying track and interacts with one or more vacuum boxes so that a negative pressure is formed on the load-carrying side of the conveying track in its perforated region or regions, a first compression arrangement through which the first conveying track runs and which is arranged so as to compress a body conveyed on the first conveying track in those parts of the body which are located in unperforated regions of the first conveying track, a second conveying track which is perforated in one or more regions extending in the running direction of the conveying track, which regions correspond in the transverse direction to the unperforated regions of the first conveying track, and interacts with one or more vacuum boxes so that a negative pressure is formed on the load-carrying side of the second conveying track in its perforated region or regions, and a second compression arrangement through which the second conveying track runs and which is arranged so as to compress a body conveyed on the second conveying track in those parts of the body which are located in unperforated regions of the second conveying track.
In a preferred embodiment, the first and second compression arrangements consist of compression rollers, and the first and second conveying tracks consist of transfer wheels.